The communications needs of mobile machines, particularly mobile machines in working environments, are becoming increasingly more complex as the work related expectations of the machines increase. For example, in the construction and mining industries, mobile machines must receive command related information, and position related information that needs to be increasingly more precise. This information may then be used to control actions of the mobile machine. If the mobile machine is autonomous, the information that is received by the mobile machine is even more important for proper operation of the machine.
The need to receive this diversity of information requires several pieces of communications equipment. The position related information may require a GPS antenna and receiver for receiving GPS signals, a differential GPS (DGPS) antenna and receiver for receiving a DGPS signal, and an antenna and receiver for receiving signals from pseudolites. The command related information may require additional antennas and receivers for receiving this data.
Attempts have been made to design and build antennas that package more than one antenna into a single unit. For example, in U.S. Pat. No. 5,691,726, Nichols et al. disclose an antenna combination that incorporates a GPS antenna and a DGPS antenna into one enclosure. However, additional antennas may still be required for receiving command information and pseudolite signals. In addition, even though the GPS and DGPS antennas are packaged into one enclosure, they are still two separate antennas and require separate preamplifiers and tuning networks, as well as diplexers to combine the two received signals.
In U.S. Pat. No. 5,625,363, Spilker discloses a system in which message signals are tramsmitted via a satellite uplink to available GPS satellites using the L3 transmission channel that is available. The messages are then transmitted by the GPS satellites back to earth, where they can be received by equipment that is in the propagation path of the transmitting satellites. This system may work well for specific applications, such as relaying data to receivers at geographically distant locations, provided the original transmitter and the intended receivers are both in view of the same GPS satellites. However, the system requires expensive satellite uplink equipment which must be directed toward the intended satellite relays. Additionally, many applications, e.g., work at a mining or construction site, is confined to a small geographic area, and does not require a system designed to send signals over vast portions of the earth.
The use of pseudolites has become increasingly popular at sites where precise and uninterrupted position signals are required. For example, at a mining site, the rough terrain caused by digging deep into the earth results in areas that are shadowed from GPS satellite signals. The proper placement of pseudolites to cover these shadowed areas is necessary to insure constant and precise position determinations of the mobile machines at the site. Sheynblat et al., in U.S. Pat. No. 5,646,630, discloses a network of pseudolites used to provide the required minimum of four GPS signals to enable precise position determination of mobile machines. These pseudolites, as disclosed by Sheynblat et al., operate at the same frequency as GPS, and use signals that are compatible with GPS antennas and receivers. Therefore, the pseudolites do not require any equipment for reception other than the receivers used to receive the actual GPS signals. However, the mobile machines at the site must still use additional antennas and receivers to receive command related signals.
The present invention is directed to overcoming one or more of the problems as set forth above.